The present invention relates generally to a load analysis system for use with an electrical utility power system, and more particularly to a load analysis system for distinguishing high impedance, low current faults from other normal events and activities on the power system. High impedance faults may be caused by, for example, downed, broken, tangled or dangling power lines, trees contacting the power lines, and various overcurrent fault situations.
High impedance, low current faults are more difficult to detect than permanent overcurrent faults, such as when a transformer fails. Conventional overcurrent protection devices have time delays which allow a temporary fault to clear, and if the overcurrent fault persists only then does the device deenergize the power line. High impedance, low current faults may initialize the timing circuits of the overcurrent protection devices but, by the end of the delay, the high impedance nature of the fault limits the fault current to a low value. The overcurrent protection devices cannot distinguish this low fault current from the levels of current ordinarily drawn by customers, so the lines may remain energized even though a conductor has broken.
Other methods of detecting high impedance faults have focused on detecting third harmonics generated by the arcing behavior of the high impedance faults. These earlier methods use detection algorithms having variations in harmonic current as the detection parameter. For instance, U.S. Pat. No. 4,851,782 to Jeerings detects high impedance, low current faults by analyzing third harmonic currents on the power lines.
By relying only on the arcing behavior for detection, high impedance fault detection systems using such methods experience significant reliability problems. These systems lack security against a false trip, causing unnecessary blackouts for the utility's customers. For example, a tree limb momentarily touching a power line may cause a momentary fault, which is cleared when the tree limb moves away from the power line. These earlier systems may misinterpret this momentary tree contact as a permanent high impedance fault, and in response cause breakers to trip to deenergize the line. Such systems may also interpret normal switching actions of the power system protection equipment as a permanent high impedance fault and cause unnecessary trips.
A primary goal of electrical utilities is to minimize such false fault detections. Most utilities need a load analysis system which detects and deenergizes a power line only for hazardous faults, such as when a broken conductor is on the ground. During other minor fault conditions, such as a broken power line dangling out of the reach of the public, it may be desirable to leave the power line energized. Although a dangling power line is hazardous, service interruptions to the electric utility's customers can also pose significant safety problems.
Thus, a need exists for an improved high impedance fault detection system for electrical power utilities which is directed toward overcoming, and not susceptible to, the above limitations and disadvantages.